The invention relates to new glasses which are particularly suitable for solid state laser amplifier applications.
The most powerful coherent light impulses hitherto known are generated by solid state lasers, in which the impulse of a pilot laser is amplified through a chain of amplifier stages. Such an amplifier stage primarily comprises a laser glass rod containing an effective amount of activator ions, e.g. Nd.sup.3+, which are excited to fluorescence by the light of flashlamps. This energy of excitation is surrendered by induced emission to the laser beam as it travels through the rod. After passing through a large number of such amplifiers, the radiation intensity can become so great that the resulting high electric field intensity displaces the electrons in the shell of the atoms of the glass so strongly that the refractive index is thereby increased.
In electric fields of moderate intensity, the dielectric displacement is proportional to the electric field intensity: EQU D=.epsilon.E
wherein:
D=dielectric displacement PA1 E=electric field strength PA1 .epsilon.=dielectric constant.
For extremely high electric field intensities, this linear relationship no longer applies and a quadratic relationship appears: EQU D=.epsilon.E+.epsilon..sub.2 E.sup.2
The root .cent..epsilon..sub.2 =n.sub.2 is a measure of a so-called "non-linear effect." The quantity n.sub.2 is therefore called the non-linear refractive index and is expressed in electrostatic units. Increasing the refractive index causes a focussing of the beam. The intensity is greatest in the interior of the beam cross-section and diminishes towards the marginal zone of the beam cross-section. The non-linear refractive index also has a corresponding distribution and a convex lens effect is also obtained by this means. It is important that the glass which is used in the amplifier stages should have a low non-linear refractive index in order that self-focussing does not become so intense that an electric discharge is generated in the glass, which would destroy it. The glasses of which are made the lenses for beam spreading or beam focussing, as well as the plates which are present in the beam path in order to reflect out a small part of the laser beam for measurements, must likewise have a low non-linear refractive index, i.e. the glasses, even without incorporated activator ions, have great significance in the construction of laser amplifier chains.
It is known, e.g. from German Pat. Nos. 1,171,082 and 1,255,215, to produce laser materials based on silicate glasses, which provide a good fluorescence life for the Nd.sup.3 ions (up to 1000 microseconds). Such silicate glasses can be produced in optically homogeneous form in large volume and the number of absorbent particles in the glass can be kept small. The disadvantage of such glasses is their relatively high non-linear refractive index and the corresponding limited effective cross-section of induced emission.
It is also known according to U.S. Pat. Nos. 3,250,721, 3,846,142, and French Pat. No. 2,291,949, to produce laser glasses based on phosphate glasses. These glasses have large effective cross-sections of induced emission, they can also be produced with good optical homogeneity in large volume and their content of absorbent particles is low. However, the fluorescence life is restricted to approximately 300 microseconds and the non-linear refractive index cannot be reduced below 1.times.10.sup.-13 esu. Due to their relatively high optical refraction, said phosphate glasses do not have the desired low non-linear refractive index n.sub.2.
It is known that glasses display a reduction of the non-linear refractive index n.sub.2 with decreasing color dispersion and refractive power. This is the case with the group of fluorine phosphate glasses. High-fluoride glasses are required even for glasses with n.sub.2 &lt;0.8. Such glasses are known, e.g. see German Pat. Nos. 945,408 and 1,596,877; British Pat. No. 1,405,717; and commercially available glass type FK 51, Schott Catalog 3060/72. Such glasses contain primarily fluorides and phosphoric acid and have a far stronger tendency to crystallization and segregation than conventional silicate, borosilicate, silicoborate and borate glasses. As these known glasses display no laser properties, it is necessary to use for this purpose special laser activator elements from the rare earth group, e.g. Nd.